work flow does not start automatically first time when any item is being changed in doc library

I haved created a workflow through sharepoint designer that triggered on item changed. Now my problem is when I change any items in document library then workflow does not start first time but again when i am changing another items then workflow start.I am not getting, why workflow is not running when first time any changes in document lib is being done???????I am getting this problem on couple of day but not getting proper solution.I am using sharpoint 2010 foudation.

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